[program-java] Re: Java: Redimensioning my existing array.

  • From: "Corbett, James" <James.Corbett@xxxxxxxxxxxxx>
  • To: "'program-java@xxxxxxxxxxxxx'" <program-java@xxxxxxxxxxxxx>
  • Date: Fri, 3 Sep 2010 13:46:45 -0400

Steve:

I tried to respond but in order to eexplain, I would have to get into deep 
details. Sorry mate.

J.


-----Original Message-----
From: program-java-bounce@xxxxxxxxxxxxx 
[mailto:program-java-bounce@xxxxxxxxxxxxx] On Behalf Of Steve Meacham
Sent: September 3, 2010 13:33
To: program-java@xxxxxxxxxxxxx
Subject: [program-java] Re: Java: Redimensioning my existing array.

If you're allowed to, what exactly is this data structure supposed to 
represent/do?


On Fri, Sep 3, 2010 at 12:21 PM, Corbett, James <James.Corbett@xxxxxxxxxxxxx> 
wrote:


        Steve:

        I ended up with creating a class that contains the three values:

        Pseudo code....

        Class LineAmount {

        String province;
        String valueOfSomething;

        //getter and setters that are public
        }

        In a singlton class I have the following:

        Public Map<String, LineAmounts> MyLineAmounts = new HashMap<String, 
LineAmount>();

        And I access the new map with a public add and retreive method that 
evokes either the put or get methods of the map. I feel this is more robust.

        Jim


        -----Original Message-----
        From: program-java-bounce@xxxxxxxxxxxxx 
[mailto:program-java-bounce@xxxxxxxxxxxxx] On Behalf Of Steve Meacham
        Sent: September 3, 2010 12:42
        To: program-java@xxxxxxxxxxxxx

        Subject: [program-java] Re: Java: Redimensioning my existing array.

        Please allow me to offer a didactic review of what Java arrays actually 
are.  This will be a little bit long, so wait to read it until you have time to 
go step-by-step through what would be about a whole page in a textbook.

        // This is a single, primitive, double
        double d;  // no jokes, guys

        // This is a single dimensional array of primitive doubles double[] ad;

        // This is a single dimensional array of single dimensional arrays of 
doubles, which can be used to simulate a two dimensional array double[][] aad;

        // This is a single dimensional array of single dimensional arrays of 
single dimensional arrays of doubles, which can be used to simulate a three 
dimensional array double[][][] aaad;

        Java only supports single-dimensional arrays.  I stopped with three, 
but most Java implementations will allow you to go up to 255 levels deep like 
this.  I don't recommend it.  Each single dimensional array allows from zero to 
Integer.MAX_VALUE elements.

        Please note that it isn't until you reserve the memory that the lengths 
are specified.  This allows a lot of flexibility, such as not knowing how bit 
to make the arrays until run-time.  Now, let's reserve memory for the 
two-dimensional array in the most primitive way.

        aad = new double[5][]; // This creates an array of five arrays of 
doubles
        aad[0] = new double[10]; // The creates the first one, an array of ten 
doubles
        aad[1] = new double[2]; // The creates the second one, an array of two 
doubles
        aad[2] = new double[6]; // The creates the third one, an array of six 
doubles
        aad[3] = new double[0]; // The creates the fourth one, an array of zero 
doubles
        aad[4] = new double[5]; // The creates the fifth one, an array of five 
doubles

        Please also note that each of these sizes could have been done with a 
variable and not known until run-time.  Try to do THAT in FORTRAN!  Of course, 
Java offers what I call "syntactic sugar" for when all the dimensions are the 
same:

        // This creates an array of five arrays of doubles.  It also creates 
five arrays of ten doubles each.
        aad = new double[5][10];

        This is good and bad at the same time.  It is good because it is 
flexible and powerful.  Bad, because not only is it confusing to just about 
everybody (no, you're not alone), but because it slows things down a bit.  In 
most languages finding the value stored in aad[2][5] is as simple and fast as 
adding two times five time the size of a double to the starting address of the 
multi-dimensional array.  In most languages, most or all of this can be done at 
compile time, making it into a simple memory de-reference.  In Java, however, 
the nested arrays must be traversed at runtime, one at a time.  Finding 
add[2][5] become "retrieve and de-reference the value at add[2]; then find the 
value at add[5]."  Arrays of Objects, rather than primitives makes this even 
worse.

        Because of this, arrays are very unique and special-purposed in Java.  
It's not like in Fortran where, if you can't do it in with array, it isn't 
worth doing.  I'm not sure what problem you're trying to solve, Jim, or what 
domain you're trying to represent, but in Java, using nested-arrays is rarely 
the best solution.  The main places I find them to be appropriate is in 
graphics, cryptography, and other very specialized applications.  In business 
application programming, not so much.  There, the Collection Classes are the 
main containers for multiple instances of related same-type data, and Classes 
are the main containers for related but different-type data.

        Steve


        On Fri, Sep 3, 2010 at 10:26 AM, Willem <dwillemv@xxxxxxxxx> wrote:


               Arrays are by design not resizable. You need to copy the data to 
a bigger array.

               This is what ArrayList does behind the seens, and linkedList 
does the same for adding nodes.
               On 9/3/2010 4:24 PM, Homme, James wrote:

                       Hi Guru,

                       Maybe you can adapt the code about dynamic arrays on 
this page. It says Java can't really do dynamic arrays, but talks about a way 
to get around the problem. http://math.hws.edu/javanotes/c7/s3.html



                       Jim



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                       From: program-java-bounce@xxxxxxxxxxxxx 
[mailto:program-java-bounce@xxxxxxxxxxxxx] On Behalf Of Corbett, James
                       Sent: Friday, September 03, 2010 9:40 AM
                       To: 'program-java@xxxxxxxxxxxxx'
                       Subject: [program-java] Java: Redimensioning my existing 
array.



                       Hello all:



                       I have an array of type double....



                       Double[][] myDoubleArray = new double[2][13];



                       ...I now need to add an additional field.... Should I 
redimension the array like double[2][13][5] or should I invest in either an 
arrayList or HashMap?



                       Jim



                       James M. Corbett



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